MX Record for subdomain

Unfortunately, Live Chat seem to have no idea what I'm talking about. @bobpullen - hopefully you can help based on other topics I've seen here?

I am trying to setup an inbox and noticed it has given me myusername.plus.com as a mailbox - great. I noticed there is an option to have the mail for this go to an external SMTP server. I have assigned an IP in the control panel I want to manage mail for this subdomain but myusername.plus.com does not return an MX record. I've been querying Google's public DNS and even put a cache flush request in for my subdomain on MX and A but it is still not returning results.